Naturally occurring erbium (68Er) is composed of six stable isotopes, with 166Er being the most abundant. Radioisotopes have been characterized with from 145Er to 175Er, all having half-lives less than ten days: the most stable are 169Er, 172Er, and 160Er. All of the remaining radioactive isotopes have half-lives that are less than 11 hours, and the majority of these have half-lives that are less than 4 minutes. This element also has numerous meta states, with the most stable being 149m1Er.